Off-the-road vehicles are vehicles with two or three or more driving axles. The performance profile of such vehicles is mainly designed for off-the-road travel. In these vehicles one of the axles, usually the front axle (or front axles, if more than one), can be driven either permanently or by being switched into line. Therefore, the corresponding coupling is employed either to switch the front axle drive into line or for locking the central differential. Furthermore, one coupling is provided in these vehicles for the locking of each axle differential in order to achieve full off-the-road capability. The couplings may be disk couplings, as well as positively engaging couplings. In heavy vehicles the latter are preferred because of the high torque to be transmitted and the limited availability of installation space.
Steering an all-wheel vehicle off the road is an art which consists not the least in the ability to take the correct action in the drive system depending on the driving conditions and the nature of the terrain. These actions include connecting the front axle drive and actuating the longitudinal (inter-axle) locking mechanism or differential and the transverse (intra-axle) locking mechanism or differential. For this reason, different actions are taken by the driver in known all-terrain vehicles. Therefore, individual switches, or at least one switch with several switching positions, are provided. A rigid sequence of actions must then be followed. However, this sequence of actions does not help the driver in judging the state of the vehicle and in selecting the drive mode.
Automatic locking differentials which lock dependently of the current rotational speed differences are also known. However, if several differentials are installed in series, a problem arises as described through the following example. Differentials installed in series include first a longitudinal inter-axle differential and then intra-axle differentials connected in the direction of torque flow.
If the right rear wheel slips in a vehicle having such a series differential arrangement, the action of the non-locked differentials causes all the rotational speeds in the drive train to change. The difference between the rotational speeds of the two rear wheels is greatest. The difference between the median rotational speeds on the front and rear wheels, i.e., the average produced by the axle differentials, is smaller. This difference in speeds leads to a locking of the rear axle differential which results in only one driving wheel being available which will easily lose road adherence since the right wheel is slipping. The longitudinal locking mechanism or differential is released only when it slips and three wheels are driving, insofar as the vehicle has not become mired by then. Furthermore, when the wheels of an axle are slipping, the danger that they may laterally push away exists.
The correct action in this situation is to lock the longitudinal differential first. But if the switching threshold of the longitudinal differential is put lower than that of the transverse differential, the longitudinal differential will be locked first. However, this will cause the rotational speed difference of the rear wheels to be reduced and the switching threshold for the locking of the rear axle transverse differential will no longer be attained.
If the vehicle is equipped with a connectable front wheel drive instead of a longitudinal locking mechanism, the above described effect becomes even more apparent. The individual rotational speed differences will depend on the switching state of the couplings. Because the switching state of the couplings depends on rotational speed differences while the rotational speed differences depend on the switching state, an erratic back and-forth switching will arise if individual clutches are automatically operated individually.
Furthermore, automatically locking differentials are as a rule locked, although usually only in part, by disk couplings which, aside from their known disadvantages, are subject to much greater wear under such conditions.
If the rotational speeds of all the wheels are taken into consideration in an automatic transmission and if these speeds are processed centrally, very complicated and intricate logical criteria are needed. However, these criteria are still unable to cover all possible situations. Since the rotational speeds of the wheels depend on the switched state of the individual couplings in this case too, the danger of undesirable switching still exists, particularly undesirable and dangerous switching back into the unlocked state.
A system is disclosed in DE-C 35 05 455 and the publication AUTMOBIL-INDUSTRIE 1/87 (pages 27 to 32, dealing with the 4-MATIC of MERCEDES-BENZ). This known system automatically actuates the connection of the front axle drive, the central locking mechanism and the rear-axle locking mechanism. However, its performance profile strongly emphasizes the requirements of a fast road vehicle and is geared for safety. It is basically different from a system designed for off-the-road travel. In this system, an automatically locking differential is provided for the rear axle. The control of the front axle coupling and the longitudinal differential is based only on the axle speeds. Furthermore, this known system is dependent on the smooth engagement of disk couplings because of the safety reasons mentioned earlier.
A control system for the control of individual couplings is disclosed, for example, in the assignee's EP-OS 510 457 (U.S. Pat. No. 5,335,764). In this system, for the control of a positively engaging coupling, with all of the applications mentioned above being possible, rotational speed differences are compared with threshold values. The special disclosed systems function with actuating elements that are simple in their action and require no uncoupling command, but automatically uncouple when the transmitted torque is small enough. The instant invention has the coordination of such individual systems as its object and is applicable to the systems described therein, but is in no way limited to them. It is also suitable for any positively engaging coupling controlled in any manner and even for disk couplings.